Assessor macneilli
Electric-blue and barely 4cm, the blue assessor hides in Queensland's reef caves year-round in 25 to 29C water. Here's where to look.
A tiny fish, only about 4cm long, glowing an almost fluorescent electric blue all over its body. It has a compact, slightly elongated shape and is most often seen hanging upside down or at odd angles under ledges and in small caves rather than swimming openly over the reef. There's no bold pattern to key on here, the uniform neon blue colour against a dark cave background is the giveaway.
Look for it tucked into crevices, overhangs and small cave mouths, often hovering upside down against the roof of a ledge rather than out in the open. It stays close to shelter and doesn't range far from its hiding spot, so divers tend to spot it by peering into dark recesses rather than seeing it cruise past. Logged as uncommon at all three sites where it's been recorded, so finding one takes a bit of deliberate looking rather than a chance encounter.
Its habit of resting upside down under ledges is a real identification shortcut underwater, few small reef fish orient themselves this way as a default resting posture.
No clear seasonal pattern shows up in the sighting data, and with a comfortable range of roughly 24.6 to 28.7C it's likely present on Queensland reefs year-round. Plan around good visibility and calm conditions rather than a particular month.
All logged sightings so far come from Queensland, Australia, making it the place to look for this species on the Great Barrier Reef and adjacent reef systems. It hasn't turned up in diver logs outside Australia yet, so treat it as a Queensland speciality rather than a widespread tropical find.
Mapping where Blue Assessor is logged…
All current diver sightings are from Queensland, Australia, where it shelters in caves and under ledges on reef sites. It hasn't been logged elsewhere yet, so Queensland is the reliable bet.
There's no detected season in the sighting data, and its temperature range of roughly 25 to 29C suggests it's around year-round on Queensland reefs. Dive whenever conditions and visibility suit you.
Look for a small, uniformly electric-blue fish about 4cm long hovering upside down under ledges or in small caves. The all-over neon blue against a dark crevice background is the main clue, with no other markings to check.
They stay tiny, reaching only about 4cm in length, so divers need to look closely into cave mouths and overhangs rather than expect an easy spot.
No, they pose no danger to divers. They're small, shelter-dwelling fish that keep to themselves in reef crevices.
It regularly rests upside down against the ceilings of small caves and ledges, an orientation that's unusual among small reef fish and makes it easier to recognise once you know to look for it.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
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